Downy Mildew Risk in Willamette Valley
Plasmopara viticola · Willamette Valley AVA · OR
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Updated Jul 19, 2026, 8:48 AM UTC
Modelled forecast
7-day treatment window
Days in red are the strongest candidates for a protective spray. Index from GrapeFlow’s operational model on Open-Meteo weather — a planning aid, not a substitute for scouting.
About Willamette Valley
The Willamette Valley is cool and marine-influenced, with wet springs and autumns bracketing a relatively dry mid-summer. The damp shoulders of the season and high humidity make both powdery and downy mildew real, recurring threats — downy notably more than in California.
Signature varieties
The biology of downy mildew
Downy mildew is caused by Plasmopara viticola, an oomycete (a "water mould", more closely related to algae than to true fungi) and an obligate parasite of the grapevine. It overwinters as thick-walled oospores in fallen leaves and the top layer of soil. In spring, once those oospores have matured, the classic "3-10 rule" applies — about 10 mm of rain at temperatures near or above 10°C on shoots that have reached ~10 cm lets them germinate and splash sporangia onto the lowest leaves, releasing swimming zoospores that infect through the stomata.
Free water is non-negotiable: the zoospores must swim, so infection requires rain or a long spell of leaf wetness. Infections first show as translucent "oil spots" on the upper leaf; on warm, humid nights — relative humidity near saturation, mild temperatures — the pathogen sporulates as a downy white felt on the underside, launching a polycyclic secondary cycle that can race through a canopy in a wet season. Temperatures above roughly 30°C kill the sporangia and zoospores, so hot, dry spells stall the disease.
The vine is most vulnerable from pre-bloom through fruit set, when young leaves and clusters are highly susceptible; berries gain ontogenic resistance a few weeks after fruit set, though leaves and the rachis remain at risk all season.
How the risk index is calculated
The live risk uses GrapeFlow's structured, Rossi–Caffi-inspired downy mildew model on hourly Open-Meteo weather. It is deliberately not a full Rossi–Caffi implementation — that needs oospore-cohort tracking and soil moisture that free weather data cannot supply — so it estimates the two things the hourly series can support: primary-infection readiness and secondary pressure.
Primary readiness blends a cardinal-temperature response (rising from 4°C, peaking near 22°C, cut off by 30°C) with rainfall (meaningful from ~2 mm) and leaf-wetness duration (from ~4 hours), weighted so the weakest of the three drags the score down — all three must converge for a primary infection to be plausible. Secondary pressure counts night-time hours favourable for sporulation (relative humidity ≥85%, temperature ≥11°C), daytime hours in the 10–25°C optimum, and the length of unbroken wet spells. Two hard gates then apply: with no real free water (essentially no rain and short wet streaks) the score is capped low, and three or more hours above 32°C — lethal to the pathogen — cap it as well. Like the powdery index it is a spray-timing and scouting aid built on modelled weather, so confidence is capped at medium and lower for forecast days.
Frequently asked questions
What weather favours downy mildew?
Mild, wet weather. Primary infections follow the "3-10 rule" — roughly 10 mm of rain at 10°C or more on shoots about 10 cm long — and the secondary cycle thrives on warm, humid nights (10–25°C with relative humidity near saturation) that let the pathogen sporulate. Free water from rain or prolonged leaf wetness is essential; dry heat above ~30°C shuts it down.
Why does downy mildew need rain when powdery does not?
Because downy mildew is an oomycete — a water mould — whose spores release swimming zoospores that must move through a film of free water to reach and infect the leaf's stomata. No liquid water, no infection. Powdery mildew's conidia germinate from humid air alone, so it can build even in dry spells; the two diseases essentially favour opposite weather.
How is this risk index calculated?
It is a structured, Rossi–Caffi-inspired model run on hourly Open-Meteo weather. It scores primary-infection readiness (temperature, rainfall and leaf-wetness converging) and secondary pressure (humid nights, optimal temperatures and long wet spells), with hard caps when there is no free water or when heat exceeds ~32°C. It is a decision aid for timing treatments — not a substitute for scouting.
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